Patterns corresponding to complex circuitry are formed on wafers using photolithography. The present invention relates generally to photolithographic processes. More specifically the invention relates to a reticle or plate which is used in the photolithographic process and a method for effectively removing sources of heat which contribute to magnification and focus deviations.
As the semiconductor market pushes to resolve problems associated with providing smaller critical dimensions, photolithographers are considering ways to remove sources of heat that contribute to focus deviations and registration errors. One such source of heating occurs in the reticle or plate due to the absorption of an actinic light source into the mask. Heating of the plate causes expansion of the glass and optical components of the projection lens which contributes to magnification errors (1-2 PPM) and focus errors (50-100 nm).
Several variations to the photolithographic process are well known in the art. For example, positive or negative resist can be used. A positive resist will form an image with the same polarity on the wafer as the image on the reticle, whereas a negative resist will form an image with opposite polarity on the wafer. Although only one photolithographic method will be described below, it is to be understood that the present invention can be used in a variety of photolithographic processes.
FIG. 1 represents a number of the steps involved in a typical photolithographic process. A substrate 10 to be etched or implanted is provided over a wafer 12. A photoresist layer 14 is provided on the substrate 10. The photoresist layer 14 undergoes chemical changes when exposed with photons, such as from an ultraviolet light source 15. A mask 16 is placed over the photoresist layer 14. The mask 16 includes a stiff quartz, sometimes referred to as a plate or substrate 18, and a chrome image or pattern 20 on one side of the plate 18. Attenuated materials, may be used for attenuated Phase-Shifting Mask technology (PSM). The chrome image or pattern 20 is on the side of the plate 18 closest to the photoresist 14. The pattern 20 includes reflective portions as well as open portions. When ultraviolet light is directed from a light source 15, toward the mask 16, reflective portions of the chrome image 20 reflect the ultraviolet light while the ultraviolet light passes through the open portions of the chrome image 20. The ultraviolet lights next passes through a lens 21 positioned between the mask 16 and the photoresist 14. The ultraviolet light exposes the photoresist layer 14. The portions of the photoresist layer 14 which are exposed to the ultraviolet light undergo a chemical reaction. In this manner, the photoresist layer 14 is selectively exposed. The wafer 12 is then removed and a developing process is used to remove the unexposed photoresist 14. Upon removal of the unexposed photoresist, portions of the substrate 10 are exposed and then implanted or etched away. Finally, the remaining photo-resist 14 is removed.
FIGS. 2-4 represent a more detailed diagram of the mask 16 of the prior art. The mask 16 includes a plate or substrate 18 and a chrome image 20. FIG. 2 shows a prior art mask 16 mounted on a reticle support/alignment fixture 22 on a stepper. As best shown in FIGS. 3 and 4, the mask 16 includes a plate 18 which is typically made from quartz and a reflective image 20 which is typically chrome. The plate 18 includes an upper surface 18a and a lower surface 18b. The image 20 is located at the lower surface 18b of the plate 18. The image 20 includes reflective portions 22 and open portions 24. The reflective portions 20 and the open portions 24 form a pattern for selectively exposing the photoresist 14. By positioning the reflective image 20 proximate the lower surface 18b of the plate 18, the image 20 will be proximate the photoresist layer 14.
As shown in FIG. 4, when the mask 16 is exposed to an energy source, such as for example, the actinic wavelength, represented by arrows 26, passes through the upper surface 18a of the plate 18 and then encounters the image 20. Energy 26 passes through the open portions 24 of the image 20 and is reflected by the reflective portions 22 of the image 20. The energy 26 which is reflected by the reflective portions 22 reflects back through the glass 18 of the plate 16. The portions of energy 26 which encounter an open portion 24 of the image 20 transmit through the mask 16, with only a small amount of the energy 26 being reflected. As the energy 26 passes through the open portions 24 of the mask 16, a chemical reaction occurs in the photoresist 14.
The pattern formed by the reflective portions 22 and the open portions 24 of the image 20 effectively determines a pattern of the copper film 12 which will remain on the wafer. Based on the type of mask used, the amount of energy passing through the mask will change by a factor of up to two. In masks which have mostly reflective portions/chrome portions, i.e., “dark field,” a very high percentage of the energy will pass back into the mask. Thus, these “dark field” masks result in a greater energy loss (or absorption in the reticle or plate 16). This energy absorbed by the reticle is difficult to remove due to the bulk of the reticle or plate 16. A recently published paper documents findings that the dark field portion of an advance reticle 16 can increase in temperature by two degrees more than a light field portion of the same reticle 16. Thermomechanical Modeling of the EUV Reticle During Exposure, Carl J. Martin et al., Emerging Lithographic Technologies V, SPIE Vol. 4343(2001) page 515-523.
Masks made with absorbing materials, such as those used in attenuated PSM or embedded-attenuated PSM will absorb almost all of the attenuated light (approximately 94% for a 6% attenuated mask) in the form of heat. This heat creates large focus and magnification errors.
Another problem which is encountered using the current photolithographic processes is the presence of flare. Flare is a light scattering effect at the mask plane.
As the industry moves to both new types of “glass” (CaF2 is now being planed for 157 nanometer node) and higher energy light sources (193 nm, 157 nm, EUV), increased heating of the reticle 18 will be experienced resulting in even greater magnification and focus errors. In addition, the pure transmission of the optical components will be diminished, creating compaction in the mask and producing even greater sources of heat over time. This will be a burgeoning source of error for production of advanced semiconductor devices in the future.